[Gluster-users] Gluster FS Native Client Behaviour (3.7)

[Kaushal M]

On Wed, Feb 3, 2016 at 8:03 PM, Peter Spain <pspain at gmail.com> wrote:
> Hello
>
> I am very new to GlusterFS and have been playing around with over the last
> few weeks, with a view to using it in production. So far I found Gluster to
> be very interesting and easy to get along with. However, there seems to be a
> giant hole where the Gluster Native Client documentation should live.
>
> After using it for a few weeks and playing around (inside various VMs) I am
> still not entirely sure how the client behaves.
>
> From network captures it is clear that the client communicates to all the
> nodes for a particular volume, and that the client gets this information
> from a volfile (which it retrieves when mounting a volume). Various blog
> posts confirm this and go on to mention that the client is responsible for
> replicating data across nodes, and not the nodes themselves. I assume this
> is still the case?

Yes. This is how GlusterFS works.

>
> Beyond that I really have no idea how the client behaves in a replicated
> volume. My questions are:
>
> There is a "ping-timeout" option to adjust how long it takes the client to
> connect to a different node, in case of node failure. If the client knows
> about all nodes and actively communicates with all of them, why does it need
> a time out at all?

'ping-timeout' defines the time the client will wait for a server to
reply, before assuming the node is down and disconnecting from it.

>
> Why does the client "stick" to a particular node?

When using replication, the client reads from the first node that
responds. So it sticks to this node for further reads. I'm not sure if
the replicate xlator does the selection for each file operation or
not.
But for writes, the client makes sure writes happen to both servers
before replying to the applications. So if one of the servers isn't
responding, the operation doesn't return till the ping-timeout
happens.

>
> Does the client go back to the original node once it recovers?

If the original node comes back up, and replies quicker than the other
nodes, the client will use it.

>
> Is it possible to dictate which node a client will initially connect to on
> mounting a volume?

When you mount a volume, you give the address only for fetching the
volfile. The client then connects to all servers in the volfile.

>
> If all this information is contained in some documentation I would love to
> be pointed to it, as so far I cannot find the answer to these questions.
>
> Regards
>
> Peter
>
>
>
>
>
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